Power supplies for microwave applications are generally known in the art. As shown in FIG. 1A, the ferroresonant power supply circuit includes a power step-up transformer having a primary winding connected to a standard 120-volt AC, 60 Hz power source. The secondary circuit is connected to a voltage doubler. The voltage doubler includes a capacitor having a first terminal connected to the secondary winding of the transformer, and a second terminal connected to a rectifying diode. The output of the voltage doubler is supplied to the magnetron connected in series with the second diode.
While the above design for microwave applications is simple and cost-efficient, it is vulnerable to a high level anode current generated when the magnetron starts conducting. At the start of conduction, the magnetron presents a dynamic short circuit due to the instantaneous rate of change of voltage. Consequently, the ferroresonant power supply of FIG. 1A saturates, sharply reducing its output power level. After this initial period of conduction and settling of the anode current, the ferroresonant power supply returns to its normal output power level.
In addition to the undesirable drop in the output power level resulting from the high level anode current, the magnetron and the components of the power supply can be easily damaged by the current exceeding their specifications. The problem is particularly pronounced in UV curing operations where the power to the magnetron is rapidly turned on and off at a line frequency of approximately 8 to 10 milliseconds in order to improve the curing process.
One suggested solution to eliminate the high level anode current involves the insertion of a multiple-turns inductive coil into the circuit of the secondary winding, as shown in FIG. 1B. The coil is connected in series with the secondary winding of the transformer. As well known in the art, an inductive coil is equivalent to a virtual open circuit in high frequency AC circuits. According to the electro-magnetic properties of the coil, a voltage across the inductor is equal to the time rate of change of the magnetic flux generated by that inductor. As the rate of change of current increases, the voltage is developed across the terminals of the inductor with a polarity opposing the current through that inductor. The more rapidly the current changes, the greater is the voltage that appears across its terminals. Consequently, the nearly instantaneously occurring anode current would not flow through the electronic components of the circuits because the coil generates a voltage drop almost equivalent in magnitude to the secondary winding voltage, and a polarity which is reverse to the voltage polarity across the secondary winding. This solution, however, requires a very costly and bulky inductor. Many applications prohibit implementation of such inductor in the power supply circuit where cost efficiency and compactness are at a premium.
Another solution for reducing the component-damaging current proposes a phase control circuit in the primary circuit of the transformer, as described in U.S. Pat. No. 3,780,252 to Crapuchettes. The phase control circuit determines the phase angle of the AC voltage, supplied to the transformer from the power lines, during which the AC voltage is at a minimum level. The level of the anode current is therefore minimized as much as possible. In Crapuchettes, the phase angle of the AC voltage is selected so that the generated current does not exceed the rated specifications of the electronic components in the circuit. The control circuit monitors the phase of the AC voltage to control switching of the power source, thereby controlling the current. The disadvantages of this approach include increased complexity of the circuit and a number of additional components demanding a higher cost for the product.
In addition to suppressing the high level anode current, the power supply must provide a variable output power to the magnetron. Advances in the UV curing applications have shown that improved product quality can be obtained with the ability to continuously vary the power output. Variable power allows for much finer control and also provides the ability to compensate for any output degradation over time.
In the prior art, a phase angle control cannot be used to vary the output power in the ferroresonant circuit of FIG. 1A. The phase angle control causes the transformer in the ferroresonant circuit to saturate and produce high level currents which damage the components.
One of the solutions to the need for variable power delivery to the magnetron is duty cycling, as described in U.S. Pat. No. 4,620,078 to Smith. In Smith, a particular power level is selected by switching on or off the high voltage transformer for a number of line cycles using a triac. According to Smith, on/off cycles can range from 1 second to 30 seconds in the microwave oven industry.
The duty cycle with the microwave powered lamp, on the other hand, can be no more than 1/2 60-Hz line cycle: 8 to 10 milliseconds. If the off time is longer than 8-10 milliseconds, the bulb plasma, contained in the lamp, would extinguish. Restarting the bulb plasma then becomes extremely difficult until sufficient additive has condensed. This operation can take 10 seconds or more and is clearly impractical in the UV applications. A need therefore exists for a variable power supply in all heating applications, including UV.
Yet another desired characteristic of power supplies for magnetrons is prevention of moding. The filament is a source of electrons in the magnetron. If heated the filament produces electrons generating RF emissions. Moding of the magnetron occurs when its filament temporarily becomes depleted of electrons and stops conducting current through the magnetron. After accumulating enough electrons, the filament starts conducting again. This results in a faulty condition of the magnetron conducting current in bursts.
As the magnetron ages, the filament becomes depleted and can no longer support the electron flow required to maintain the desired power. When this condition occurs, the voltage of the magnetron jumps to a higher level to maintain the same power. When the filament accumulates enough electrons to support the required current, the voltage returns to a normal operating level. These oscillations between normal voltage/normal current and high voltage/low current damage the power supply components and cause the magnetron to operate outside its normal operating condition. When the frequency of oscillations increases, the magnetron and the power supply components can no longer perform according to the desired specifications and must be replaced.
A need, therefore, exists for preventing the moding of the magnetron and extending its operating life.